Semiconductor devices are very small, typically from 5 mm×5 mm square to 50 mm×50 mm square, and typically comprise numerous sites for the bonding of electrical conductors to a semiconductor substrate. Each bond consists of a solder or gold ball deposit adhered to the substrate. Very thin wires, usually about 0.025 mm in diameter, may be embedded in the ball deposits.
It is necessary to test the bond strength of the bonds, in order to be confident that a particular bonding method is adequate. Because of the very small size of the bonds, tools used to test the bond strength of these bonds must be able to measure very small forces and deflections accurately.
There are several different types of bond tests that are used to test bond strength. For example, shear testing tests the shear strength of a bond by applying a shear force to the side of the bond and shearing the bond off the substrate. Pull testing tests the pull strength of the bond by pulling a wire embedded in a ball deposit away from the substrate. In a push test, a force, or load, is applied in the vertical plane directly downward onto a bond.
Machines that perform these tests typically comprise a bond test tool, be it a shear test tool, push test tool or a pull test tool, that can be positioned relative to the bond under test and then either the bond or the tool are moved in order to perform the test by measuring the force needed to break the bond.
As mentioned above, in these tests it is necessary to be able to measure very small forces and deflections. Positioning of a test tool is typically achieved using some form of screw and nut rotational drive assembly. For example, a test tool may be mounted to an assembly of components that includes a nut which moves along a threaded screw when the threaded screw is driven by a servo motor. This mechanism may be used for positioning the tool correctly prior to a shear, push or pull test and may be used to drive the test tool during a pull test.
When using a screw and nut arrangement for providing the movement of the test tool towards and away from the substrate, inevitable clearances must be provided between the mating components of the screw and nut to prevent jamming, allow for thermal expansion and manufacturing variances, etc. This clearance is referred to as “backlash”. This clearance limits the accuracy to which the test tool can be initially positioned and to which the desired position of a test tool can be accurately maintained during a bond strength test.
In prior art bond testing machines, wherein the test tool is driven up and down along the vertical axis by a screw and nut drive mechanism, a spring in tension has been positioned above the tool and used to bias the tool upwardly to close the clearance between the upper thread surfaces of the nut and the thread surfaces of the screw. This has reduced backlash when the tool is used for shear testing or push testing, because these tests cause an upward force to be applied by the tool during the test. For example, in shear testing, as the test tool shears a ball deposit off the substrate, a vertical force component results, causing the ball deposit to push up on the tool. Since the tool is already being biased in the upward position by the spring, the backlash clearance has already been closed, and thus, the shear or push test itself does not cause a tool position problem associated with backlash.
However, the use of a biasing spring has not completely eliminated the backlash problem during shear and push tests because the force applied by the spring, which as mentioned is in tension, changes as the spring is stretched. The more the spring is stretched, the greater the biasing force it applies. Therefore, a varying force is applied by the spring over the range of travel of the nut along the screw. Consequently, the spring applies more force to reduce backlash clearance when the spring has been stretched to the lower end of travel of the nut along the screw than it does when the spring is stretched to a lesser extent at the upper end of travel of the nut along the screw. Thus, the use of the spring has reduced backlash problems in shear and push tests, but it has not eliminated backlash problems.
In addition, in shear testing, it is very important that the lower end of the test tool maintain a very small, closely controlled standoff distance from the substrate. In that the upward bias of the spring varies depending on the length of the spring, the ability of the upward bias force to close off clearances also varies, making it difficult to accurately and reliably control the standoff distance for every position of the test tool above the substrate.
In a pull testing, moreover, a more significant backlash problem exists which is not solved by the use of the spring. In a pull test, the test tool is in the shape of the hook and it hooks under a wire that is bonded to a substrate. The test tool pulls up on the wire to pull the wire off a bond on the substrate and the force required to break that bond is measured. Typically the wire has been bonded to a solder ball on the substrate. As the wire is pulled, it exerts a downward force on the test tool. This downward force is pulling the tool down against the force of the spring. At some point, this force can overcome the spring force and cause the nut to move downward away from contact between its upper threaded surfaces and the screw until it makes contact with its lower threaded surfaces and the screw, closing the backlash clearance between the lower threaded surfaces of the nut and the threads of the screw. This movement to close the backlash clearance below the nut distorts the signals provided by the strain gauges, or other transducers, that are later described, and causes inaccurate force readings.
Furthermore, in the case of shear, push and pull tests, it is desirable that the nut and screw remain engaged in contact in a constant fashion to accurately control the axial movement of the test tool in a repeatable way. This has not always been achievable with the spring solution of the prior art.